Wherever trace impurities in the order of about one part per million are detrimental, as in a scintillation phosphor, or where it is essential that a fully dense, optically integral crystalline material be formed, as for example in a laser window, no method, other than press-forging a fully dense crystalline mass will suffice. By "fully dense" and "optically integral" we refer to a crystalline material which has essentially the same optical properties as a melt-grown ingot. In particular, an optically integral or fully dense optical crystal characteristically is free of voids large enough to cause appreciable scatter of transmitted radiation, and more specifically, can transmit radiation of short wavelength in the range from about 0.1.mu. (micron) to about 1.mu., without any more scatter than a melt-grown single crystal of the same material, in other words, characterized by specular transmission in the short wavelength region, less than 1.mu.. Thus, a polycrystalline article of relatively large diameter may be formed from an ingot of relatively smaller diameter, one being optically indistinguishable from the other, except that one is essentially a single crystal and the other is polycrystalline. In particular, windows may be press-forged which permit the transmittance of laser beams with less than 0.1% absorption, or no more absorption than is displayed by the archetype monocrystalline ingot, sometimes referred to as a macrocrystal, whichever is greater. In addition, press-forged windows are characterized by several times greater strength than that obtained with essentially single crystal ingots. Similarly, optically integral scintillation phosphors may be formed having large diameters which are theoretically not limited by the size of the largest ingot which may be grown. Particularly useful ingots are melt-grown from the ionic salts of the alkali metals and alkaline earth metals, either pure or in solution one with the other; or from the ionic salts of the alkali metals and alkaline earth metals containing minor amounts of dopands which serve either to enhance the optical and scintillation characteristics of the ingot, or its strength, or both.
Single crystals and essentially single crystal ingots are used in a host of applications. For example, windows transparent to radiation in the range from microwave through infrared and ultraviolet radiation are used in instruments of various kinds, including the transmission of laser beams and infrared and ultraviolet light where the crystals are used as domes or windows in missiles and related devices, communication transmission stations and the like. Doped crystals effective as scintillation phosphors are used for the detection of ionizing radiation in conjunction with a photomultiplier tube in devices ranging from simple scintillation counters to sophisticated camera plates for medical use in connection with the analysis of gamma radiation emanating from patients who are injected with specific active isotopes.
In the application mentioned hereinbefore, as well as in many others, shaped articles transparent to the aforementioned wave length range have been limited in size by the peculiar physical properties of ionic crystals. The demanding requirements of windows, domes and lenses large enough for modern commercial and military requirements are antithetical to the well-known size limitation of shaped articles made from optically integral ionic crystals. Relatively large shaped articles have been prepared from hot-pressed powders of calcium fluoride and other alkali metal and alkaline earth metal halides. In addition to finely divided powders of polycrystalline materials in the range from about 20 to about 500 microns, single crystal fragments the average particle size of which ranged from less than 10 microns to several millimeters have also been used. The hot-pressed shaped articles formed have included lenses, windows and the like which are radiation-permeable, though with varying degrees of permeability depending upon the wavelength of radiation. Characteristically, these hot-pressed articles are incapable of transmitting radiation coherently. The deficiency of hot-pressed crystals and powders, formed into articles as described in U.S. Pat. No. 3,359,066 are known to have markedly inferior properties as compared to lenses, domes and windows made from single crystals, or macrocrystal artificially melt-grown ingots. More refined hot-pressed polycrystalline materials, with random orientations of the grains, have been made as disclosed in U.S. Pat. No. 3,453,215. The method includes many purification steps of fine powder which is then hot-pressed into a laser host material. Despite the refinements, these materials are generally not fully dense, that is, they are less dense than a melt-grown ingot and have an unacceptably high absorption coefficient for the transmittance of high-power laser beams in excess of about 1000 watts/cm.sup.2.
Macryocrystal ingots are presently grown in sizes up to about 36 inches in diameter and about 1 foot long, wherein individual crystal grains may range in size up to a nominal diameter of about 8 inches or more. Large as these ingots are, they are not large enough; more important, were these ingots large enough, they would not have sufficient strength.
Thus, as of the present time, where a high quality, optically integral shaped article is required, whether it is for use as a scintillation phosphor, laser window, or an infrared or ultraviolet transmitting lens, when a relatively large shaped article is required, several individual single crystal pieces, or sections from a macrocrystal ingot are individually sawed off the melt-grown ingot and then adhesively bonded together to form the larger shaped composite. Notwithstanding the arduous devotion which is a perquisite of successful fabrication of a relatively large shaped article having superior radiation transmission properties, the end result was an article of determined fragility, and transmission properties so marginally superior to those of hot-pressed articles as to negate the use of such relatively large composites for all but those applications where cost is not a factor. Moreover, a composite formed in this manner suffers from the drawbacks of degradation of light output due to the optical interfaces. No matter how carefully the faces of sections are polished before they are bonded into a composite, there is no way of eliminating the undesirable effects of the interface. Particularly in the case of windows for lasers, where it is essential that the radiation be maintained in coherent form, it is impractical to use a composite of macrocrystal sections. Assuming that adjacent macrocrystal sections were cleaved along matching planes, no matter how carefully the faces are polished, there would exist between the faces some macroscopic voids, however small, which would interrupt the transmission of coherent light. If the faces are adhesively bonded, the energy level of the laser to be transmitted would be predicated upon the temperature sensitivity of the energy-absorptive adhesive used. Assuming it is possible to maintain the adhesive at a relatively low temperature, there would still be the problem of matching the refractive index of the adhesive with that of the crystal. The slightest difference in refractive index would generate reflections that would destroy the usefulness of the composite as a laser window.
In pending applications Ser. No. 180,087; 179,787; 139,217 all now abandoned; and 166,725 are described the formation of scintillation phosphors formed as extrudates of unrestricted length and arbitrary cross sections by extrusion of a single crystal or macrocrystal ingot at a temperature below its melting point and under sufficient pressure to form a coherent, homogeneous, fully dense polycrystalline material; stated differently, the shaped polycrystalline scintillation phosphors are formed by a process of extrusion or extruding, defined as: "to shape (as metal, plastic, rubber) by forcing through a specially designed opening often after a previous heating of the material or of the opening or of both" (see Webster's 3rd New International Dictionary, G. & C. Merriam Company, publishers, Springfield, Mass. 1966). In a more technical sense, extrusion is defined as "shaping (metal) into a chosen continuous form by forcing it through a die of appropriate shape" (see Definitions of Metallurgical Terms, p. 6, Metals Handbook, 1946 Edition, published by The American Society for Metals).
Extrusion of normally frangible macrocrystal ingots as disclosed in the aforementioned patent applications, and in the prior art, is effected in a confined zone and is distinct from press-forging as claimed in this invention. For example, it is known that a single crystal billet of sodium chloride may be fitted tightly into an extrusion chamber and forced through an extrusion dye maintained at various temperatures above 300.degree. C. to yield a rod of polycrystalline sodium chloride which is completely clear and free from porosity and therefore, optically integral. (See "Mechanical Properties of Polycrystalline Sodium Chloride" by R. J. Stokes, Proceedings of the British Ceramic Society, Vol. 6, p. 192, June, 1966). This work was done in connection with a study of the mechanical properties of polycrystalline sodium chloride in relation to those of polycrystalline magnesium oxide which has a similar lattice structure but a melting point so high (2650.degree. C.) as to make the direct study of the "more technologically significant material", namely magnesium oxide, all but impossible. This study is not directly related to the press-forged crystals of this invention.
It is also known that an almost completely transparent polycrystalline cesium bromide disk can be made under pressure in a steel mold from a single crystal pressed at 8,000 PSI at 300.degree. C. under vacuum, heated to 400.degree. C. and then cooled to 100.degree. C. while being maintained under pressure. The single crystal was maintained in a confined mold, under radial constraint, presumably to ensure the formation of a disk without fractures. The reported study entitled "The Mechanical Behavior of Single Crystal and Polycrystalline Cesium Bromide" by L. D. Johnson and J. A. Pask, J. Am. Ceram. Soc., 47, 9, 437-444, 1964, was undertaken because CsBr has an interpenetrating simple cubic structure rather than an interpenetrating face-centered cubic structure characteristic of rock salt or lithium fluoride. The authors reported that single crystals of cesium bromide were found to be "soft" and ductile if they were favorably oriented relative to the loading axis to activate the 110 slip systems. There is no indication that any deformation of the single crystal was effected. Neither is there any indication that if there was any deformation, that the deformation of the crystal was effected without fracture, or, whether any fracture that occurred was reconstituted or `healed` under the particular conditions of temperature, pressure and constraint. Having formed an almost completely transparent polycrystalline disk from a monocrystalline disk in a confined mold, the authors studied the polycrystalline structure so formed and stated "Because of the lack of 5 independent slip systems for this crystal structure, general plastic deformation of polycrystalline cesium bromide cannot occur without formation of voids or gaps between the grains unless some other deformation mechanism can operate. Kinking, the only other possible mechanism observed, required considerably higher stress levels than were attained in these specimens" (bottom of p. 443). It is hypothesized that the lack of radial constraint in the press-forged crystals of this invention provides the basis for the `some other deformation mechanism` which permits total optical integrity and transparency, rather than almost complete transparency, whether the starting material is an essentially single macrocrystal ingot or a polycrystalline mass. This unexpected transparency or unhindered permeability extends not only to light in the infrared, visible and ultraviolet regions where it is greatly desirable, but to high-energy coherent laser beams in excess of 1000 watts/cm.sup.2 where it is a necessity.
The polycrystalline nature of a press-forged body is easily recognized by etching the crystal surface so as to show off the fine grain structure of a typically polycrystalline body.
A press-forged optical body may be distinguished from a polycrystalline extrudate, derived from a single crystal ingot, by the clear orientation of the grains along the axis of extrusion of the extrudate, while the press-forged optical body lacks such a characteristic axial grain orientation. A typical press-forged optical body shows crystallites having radially diminishing grain size and the majority of individual crystallites having an orientation radially generally angularly disposed to the direction of force used to form the press-forged body. It should be kept in mind, that the degree of radial orientation and the degree of radially diminishing grain size will probably be obscured by multiple press-forging, particularly if the direction of the applied force is changed.
The major distinction between a press-forged polycrystalline optical body which is optically integral and a hot-pressed body of the same material is the scatter at short wavelengths. There is no argument that well-prepared hot-pressed bodies are optically integral to infrared radiation, but these same bodies show much scatter, or may even be essentially opaque in the short UV range. A second clear distinction between a hot-pressed optical body and a press-forged optical body of this invention is the scatter of a laser beam. A hot-pressed body shows a fuzzy pattern while the press-forged body will show a sharply defined pattern. The more numerous the voids in the hot-pressed body, the greater the fuzziness of the pattern.
Distinguishing a press-forged optical body derived from an extrudate may be difficult depending upon whether or not the press-forging direction coincides with the axis of extrusion, the time and temperature over which the press-forging is effective, and the extent of the compressive deformation. Generally, any extrudate which has been press-forged will exhibit, by its grain structure, that the orientation of the grains along the axis of extrusion has been interfered with. Though a clearly radial and angular orientation of the grains with respect to the axis of force application during press-forging may not be evident, it would be apparent to the practiced eye, that the optical body is not one that has been extruded only.
Certain ceramic materials such as spinel crystals, magnesium oxide, calcium oxide and titanium carbide have been forged under extreme pressure and temperature conditions, as described in "Deformation, Recrystallization, Strength, and Fracture of Press-Forged Ceramic crystals" by Roy W. Rice in the Jour. of the American Ceramic Soc., Vol. 55, No. 2 pages 90-97. The gist of the article is that these ceramic crystals can be press-forged, that they evidence a polycrystalline structure when they are press-forged, that they have a tendency to recrystallize, and that despite generally similar behavior, it is difficult to predict performance characteristics of one material though armed with knowledge of performance characteristics of a similar material. There is a conspicuous absence of any reference herein that the ceramic crystals behave in an analogous manner compared with similarly structured lattices of ionic salts, and specifically, halides.
The work most closely related to our invention is described in "Preparation of Polycrystalline Alkali Halides by high-temperature deformation of Single Crystals" by V. Traskin, Z. Skvortsova, et al. (see Soviet Physics-Crystallography, Vol. 15, No. 4, Jan-Feb. 1971). It teaches a two-stage forging method which "comprises plastic deformation of single crystals at an elevated temperature, followed by recrystallization annealing". It more specifically states that single crystals of potassium fluoride were mounted between parallel plane faces of a press, heated to 400.degree. C., and "upset" under a load of 700 kg. to give relatively thin tablets about 6 mm. thick. The furnace was then removed while leaving the sample under the load; when it had cooled down to 300.degree. C. (after about three minutes), the load was increased to 1200 kg. to complete the deformation of this sample, the thickness of which decreased to about 4-4.5 mm. The load was taken off after about ten minutes when the sample had cooled to 150.degree. C.
The rate at which the load was applied is not stated and it cannot be determined whether or not the single crystal was fractured during the initial upsetting stage at the end of which a "tablet" was assumed to have been formed. The tablet was then permitted to cool to 300.degree. C. under load and, in a second stage, the load was increased at the 300.degree. C. temperature, which is below one-half the melting point of the potassium chloride crystal used by them. Small samples obtained showed no appreciable turbidity, but there is little to suggest the unique properties of the press-forged product of this invention.
The difficulty of predicting the effect of press-forging a single crystal, when the conditions are not well-defined, is exemplified in U.S. Pat. No. 3,794,704 to J. D. Strong where it is taught that the forging of a single crystal, which starts as a single crystal, also ends as a single crystal.